Radio receiving apparatus



(m18, 19:42.y QJ. K @HNSQN 1,883,194

RADIO vRECEIVING' AIPPARATUS Filed' July '7. 1951 ATTORN EYS Patentedkst. 18, 1,932 l UNITED STATES PATsNiT.- ori-ica JOHN KELLY JOHNSON, OFBYE, NEW YORK, .ASSIGNOB TO mm CORPORATION v RADIO BECEIVING'APPAMTUSApplication mea July 7, iezn.V serial no.

The present invention relates to a radio frequency tuning system for usein connection with radio receivingapparatus bf the superheterodyne type.l

A problem common apparatus is the/selection of the desired signal andthe rejection of undesired signals. A superheterodyne radio receiver isarticularly sensitive to not only the desire signal but to a secondfrequency known as the image'frequency which is that frequency displacedabove the local oscillator frequency by the same amount that the desiredfrequency is displaced below the local oscillator frequency. If theradio frequency tuning circuits of a superheterodyne respond tothecomparative close inxage7 frequenc and ifk a second transmittingstationis within the range of the receiver and operates at, afrequency at ornear the image frequency, the local oscillator frequency currents willcombine therewith and the intermediate frequency stages will pass thebeat frequency thus produced, resulting in a signal which may becharacterized as an unpleasant combination of the desired and undesiredsignals.

lt is the object of the present invention to prevent the response ofradio .receivers to undesired signals and particularly the production ofundesired image frequency signals in a superheterodyne receiver.

Various means have been proposed to eliminate the image frequencyresponse occurring in superheterodynes. One such system utilizes atapped secondary in the input transformer, the input to the first radiouency amplifier tubes being taken across a portion of the secondarytransformer. The portion of the secondary above the tap, in conjunctionwith the tuning condenser, constitutes a radio frequency bv-pass circuitresponsive to the undesired signal. Such a circuit is shown. forinstance, in MacDonald Patent No. 1,680,424.

to all radio receiving tion, an input transformer is provided, and

the secondary is tapped at a point such that the portion of saidsecondary above the tap,

together with the tuning condenser, will constitute a radio frequencyby-pass to signals of the image frequency and the terminals of theportion of the secondary below the tap of the undesired or imagefrequency.

As the entire secondary circuit is tuned to the desired frequency, theby-pass circuit, which includes the tuning element, is likewise tuned tothe undesired or image frequency.

`The misalignment between the actual image frequency and by-passfrequency is made as slight as possible and may be made zero for anypoint of the range desired. The voltage variations vbetween the tap andthe ground end'of the secondary are used to supply a voltage to a secondtuned circuit, which circuit is vutilized to impress the signals uponthe radio frequency amplifying devices, detectors, etc.

Having thus briefly described the invention, attention is invited to theaccompanying drawing, in which Fig. 1 shows the imput radio frequencycircuits o'f a radio receiver embodying this invention;

Fig. 2 shows the tuned radio frequency circuits, embodying thisinvention, used in con- `*will be at node potential relative tocurrentsnection with the input of a radio receiver as, for instance, asuperheterodyne; 1

Fig. 3 is an elementary circuit diagram illustratingy the principles ofthe present 1n- 5 vention; and

Figs. 4, 5, 6 and 7 are elementary diagrams of modified circuits forcarrying out the principles of the invention.

Fig. 1 shows the tuned input of a radio relo ceiver. ln this figure theantenna A is connected to the ground G through the primary P of thetransformer T1. The secondary S of the transformer is tapped at point14, said tap being connected to the primary P2 of the transformer T2. 1nshunt with the secondary S is the tuning condenser C1, said secondary Sand condenser C1 forming a tuned circuit 10 resonant to the frequency ofthe desired signals. Circuit 10 is made resonant to the desiredfrequency either by variation -of the condenser C1,made variable-asshown, or by variation of the inductance of the secondary S. Thesecondary S2 of the transformer T2 is shunted by condenser C2, formingthe tuned circuit 12 which is tuned to the desired. frequency in amanner similar to that inwhich circuit 10 is tuned.

Condensers C1 and' C2 may be connected for uni-control operation, asindicated by the dotted line in Fig. 1, and the same control may be madeto tune the remaining tuned circuits of the receiver. A

Circuit 12 is connected to the input of thermionic device 13 which maybe the radio frequency amplifier or detector of a radio receiver, asdesired.

Attention is now invited to Fig. 2 in which similar parts are indicatedby the same reference figures. This circuit is particularly adaptedfor'use in a superheterodyne radio receiver. It differs from Fig. 1,just described, principally by the use of a volumecontrol variableresister R, a variable portion of lwhich is shunted across the primary Pof the transformer T1. The cathode of the thermionic device 13 is alsovariably connected to ground thro gh the resistor R for the purpose ofvarying t e amplification of said device by varying its grid biaspotential. Transformer '1 1 includes also a second primary P1 which maybe wound on the secondary for the purpose of increasing the gain of saidtrans- I ormer by increasing the capacity coupling i between the primaryand secondary. This circuit is in other particulars essentially the sameas that shown in Fig. 1.

In view of the fact that the primary P2 of transformer T2 is inv shuntwith a substantial portion of the tuning inductance S of transformer T1of Fig. 1 and T1 of Fig. 2, it is desirable that its inductance be highin order to prevent its acting to short-circuit the portion of coil Sbelow the tap, and thus interfere with the alignment of circuit 10 withthe circuit 12.

As an example of what constants may be used for the various parts of thecircuit constituting the invention, the following characteristics aregiven, although it is to be noted that others may be found satisfactory.How- P1-20 turns of #38 U. S. gauge double silk covered copper wire;

C1 and C2--O-350 micro-micro-farads. The secondary S comprises 125 turnsand the tap is vtaken 51 turns from the ground end.

These characteristics are for a superhetero- 1 dyne receiver using anintermediate frequency of 175 kilocycles in which the image frequency is350 kilocycles above the desired frequency.

The condensers C1 vandCz may be connected together for uni-controloperation. The circuit of Fig. 3 is the same as the circuit shown inFig. 1 except that the inductances are differently arranged for thepurpose of explaining the action of the circuit used in accomplishingthe objects of this invention. As previously described, the secondary Sof transformer T1 is ta ped at point 14. The two portions Sfand S9'that, relative to the points 14 and 15, S and C1 will constitutea-"by-pass to currents of an undesired frequency. It can, therefore, beseen that the voltage gain characteristic curve of the portion S willshow zero voltage at the image frequency, whereas the natural resonanceof the circuit 10 may be such that a considerable voltage will bedeveloped at a frequency the same amount on the other side of the signalfrequency, thus producing a non-symmetrical curve relative to thevoltages between points 14 and 15 at the different frequencies. may beso chosen that the misalignment between the by-pass frequency as variedwhen the condenser C1 is varied in tuning circuit 11 and the imagefrequency will be zero at any selected point in the range. Then thepoints 14 and 15 will remain at substantially less potential differencerelative to currents of such undesired frequency throughout the tuningrange of the receiver.` The points 14 and 15 are connected to theprimary P2 of the transformer T2, thus forming a link circuit 11. It isto be noted that the primary P2 does not have impressed across it all ofthe voltage variations which are available across the secondary windingS, but only those existing across the substantially reduced portion S.However, the primary P2 is inductively related to the secondtunedcircuit are so proportioned The tap point 14 12 through the secondaryS2, the resonance characteristics of which permit the amplification tobe made the maximum obtainable in the second of two coupled tunedcircuits. In other words, substantially optimum coupling will beobtained regardless of the loss occasioned by the tapping of thesecondary. Fig. 4 shows a coupling circuit in which the link circuit 11includes a condenser C3. The o circuit 11 may be made resonant to afrequency below the band it is desired to receive, in order to increasethe response of thewhole coupling circuit at low frequencies. For thispurpose the inductance of P2 must be l5 `made relativel high, and thecoupling between P2 and g2 must be relatively loose.

Fig. 5 is a second modification of the circuit constituting the presentinvention. In this circuit the link circuit 11" includes a portion S2 ofthe inductance S2 in addition to the series condenser C3. A circuit ofthis type gives a bettery response at high frequencies in which thecurrent flowing in the linl circuit is greater. However, it is imffportant in this circuit that the capacity of C2 be small to preventshort-circuiting of the portion S of secondary S.

Fig. 6 shows a third modification of the circuits embodying theinvention. In this cirsu cuit the primary P2 of the transformer T2 isclosely coupled to the high-potential end of the resonant circuit 12.The winding P2 may be in such a direction relative to the winding of thesecondary S2 that th inherent capacity coupling C.2 between the primaryP2 and secondary S2 will either oppose orl aid the mutualinductivecoupling between said primary and secondary. In the event thatthe capacitive, coupling C4 is in opposition to the inductive coupling,the response of the circuits is reduced at the higher frequencies. Fig.7 is still another modification of the circuits embodying thisinvention. In this circuit P2 is connected to the tapped secondsary S oftransformer T1 and the other end of the primary P2 is` left unconnected.In this circuit the link circuit 11", comprises the portion S oftransformer T2, the inherent capacity C2 between theprimary P2 andsecondary S2 of transformer T2 and the leads connecting S with P2', andcircuit. 12 with circuit 10. This circuit gives better responsecharacteristics to the entire coupling circuit at the higherfrequencies. The operation of circuits shown in Figs. 4,

5, 6 and 7 is similar to the operation of circuits shown in Figs. 1, 2and 3 and may be summarized as follows: The tapped secondary S of thetransformer T1 permits a bypassing in each instancefor theI undesiredfrequency as, for instance, the image fre quency in a superheterodyne.The voltage exlistng across the portion S of said secondary. is appliedby means of a link circuit to 5,5 excite a second tuned circuit 12.Various means have been shown for coupling the circuits 10 and 12, butit is to be understoodthat they are each based upon the same principles,although the method of obtaining the desired mutual coupling may bevaried in accordance with the circumstances, as shown dfor the purposeof illustration in the various figures.

When the circuits constituting this invention are being used in asuperheterodyne -receiver, the tap 14 is so chosen that the imagefrequency response of the tuning circuits will be reduced. However, whena tuned radio frequency receiver is used the tap may be so taken thatany objectionable signal may be rejected. I claim: i fr 1. Signalselecting circuits for a radio receiver comprising a circuit :tunable tothe frequency of the current it is desired to receive, a second circuittunable to the same frequency, and a link circuit connecting portions ofthe impedances of each of said circuits, tlie portion of the impedanceof the first of said circuits being shunted by a circuit resonant to thecurrentof a frequency it is not desired to receive and said link circuitbeing nade resonant to current of a frequency lower than the band offrequencies to which v said circuits are tunable.

2. A signal selecting device for a superheterodyne radio receiver whichcomprises a circuit ltunable to a current of the frequency it isVdesired to receive, a second circuit likewise tunable to the current ofthe frequency it is desired to receive, and a link circuit connecting aportion of the impedance of said first mentioned circuit and inductivelyrelated to the second mentioned circuit, the portion of the impedance ofsaid first mentioned circuit being so chosen that its terminals remainat substantially n ode potential relative to the image frequency of saidreceiver as the circuits are tuned, said circuits being so proportionedthat the gain of said second circuit will compensate to some extent forthe loss occasioned by the tapping of only a portion of the desiredsignal voltage across the first mentioned circuit.

3. A tuned radio frequency selective device for a radio receiverincluding a circuit tunable to the frequency -of a current to bereceived, asecond circuit also tunable to the frequency of the currentto be received. and a link circuit connecting portions of the impedancesof each of said circuits, the portion of the impedance of the first ofsaid circuits being so selected that its terminals are at node potentialrelative to currents of an undesired frequency, and said link circuithaving a resonance at a frequency outside the band of frequencies tobe-,covered by said circuits whereby to emphasize the response tofrequencies toward one side of the band.

f1. Signal selecting circuits for a radio re- Y ceiver which comprise alfirst circuit includsired to receive, a link circuit connected betweensaid circuits including a portion of the inductance of the first of saidcircuits and at least a portion of the impedance of saidsecond-mentioned circuit, said link circuit being proportioned so thatthe impedance shunting4 the selected portion of said inductance remainssubstantially resonant to current of a particular undesired frequency asthe circuits are tuned, said link circuit being resonant to a frequencybelow the band of frequencies to which it is desired to tune the saidcircuits whereby the gain of said circuits is increased at lowfrequencies.

5. A signal selecting device for a super' heterodyne radio receiverwhich comprises a circuit tunable to a current of the frequency it isdesired to receive, including aninductance and a capacity, a secondcircuit likewise tunable to the frequency`` of the current it is desiredto receive, also including an inductance and a capacity, a link circuitconnected toV a portion of the inductance of the said first mentionedcircuit and inductively related to said second mentioned circuit, aportion of the inductance of said first mentioned circuit being sochosen that its terminals are at node potential relative to the imagefrequency of said receiver, and a capacity of said link circuit soselected that said link circuit is resonant to a frequency outside theband of frequencies tol Which it is desired to tune said receiver.

6. A signal selecting device for a radio frequency receiver whichcomprises a circuit tunable to a current of the frequency it is desiredto receive including an inductance and a capacity, a second circuitlikewise tunable to the frequency of the current it is desired toreceive also including an inductance and a capacity, and a link circuitconnecting a portion of the inductance of said first mentioned circuitand including an inductance which is inductively related to theinductance of said second mentioned circuit, the portion of theinductance of said first mentioned circuitV being so chosen that itsterminals are at node potential relative to a specific frequency whichit is not desired to receive.

7. A radio signal selecting circuit for a frequency as the circuits aretuned, said link circuit includin an inductance inductively related tothe high potential Vend of the inductive reactance of said second tunedcircuit whereby the coupling between said tuned circuits includes themutual inductance between the inductance of said link circuit and theinductive reactance of said second circuit and the inherent capacitybetween said inductances.

8. A signal-selecting device for a radio receiver which comprises acircuit tunable to a current ofthe frequency it is desired to receive,including an inductance and a capacity, a second circuit likewisetunable to the frequency of the current it is desired to receive alsoincluding an inductance and a capacity, and a link circuit connected toa portion of the inductance of` said first mentioned circuit andincluding an inductance of relatively high impedancewhich is inductivelyrelated to the inductance of said second mentioned circuit, the portionof the inductance lof said first mentioned circuit being so chosenthatits terminals are at node potential relative to a specific frequencywhich it is not desired to receive, said circuits being so proportionedthat substantially optimum coupling will be had between said first andsaid second mentioned circuits.

9. A signal selecting device for asuperheterodyne radio receiver whichcomprises a circuit tunable to a current of the frequency it is desiredto receive, including inductive and capacitive reactances, a secondcircuit likewise tunable to the frequency it is desired to receive alsoincluding inductive and capacitive reactances, and a link circuitconnected toa portion of the reactances of said first mentioned circuitand including a relatively high inductance inductively related yto theinductive reactance of the second mentioned circuit, the portion of thereactance of said first mentioned circuit being so chosen that itsterminals are at node potential to the image frequency of said receiver,said circuits being so proportioned that the gain of said secondmentioned ycircuit will permit optimum coupling regardless of the lossoccasioned by tapping off only a portion of the desired signal voltageavailable in said rst mentioned circuit.

10. A signal selecting device for a radio receiver which comprises acircuit tunable to a current of the frequency it is desired to receiveincluding an inductance and a capacity, a second circuit likewisetunable to the frequency of the i current it is desired to receive, andalso including an inductance and a capacity, a link circuit connecting aportion of theinductance of said first mentioned circuit and includingan inductance which is inductively related to the inductance of saidsecond mentioned circuit, the portion of the d inductance of said firstmentioned circuit be ing so chosen that its terminals are at nodepotential relative to a specific frequency which it is not desired toreceive, and a capacity in `said link .circuit so selected'that -saidlink circuit is resonant to a frequency below the band of frequences towhich it is desired to tune said receiver.

11. Radio signal selective circuits for superheterodynel radio receiverswhich com-v prise two radio frequency circuits tunable to the frequencyof the current it is desired to receive, each including inductive andcapacitive reactances, a link circuit connected across a port/ionof thereactance of one of said tuned'circuits so selected that the terminalsof said link circuit'are at node potential relative to a current of theimage frequency and including an inductance inductively related to thehigh potential end of the inductive reactance .of the second of saidtuned circuits whereby the coupling between said tuned circuits includesthe mutual inductance between the inductance of Said link circuit andthe inductive reactance of said second mentioned circuit and theinherent capacity between said inductances, the winding of saidinductances being so arranged that the mutual inductance coupling willoppose the capacitiveY coupling between the link circuit and the secondof said circuits, thus providing a lesser response at the high frequencyportion of the tuning range of said circuits.

12. A radio signal selecting device for a radio receiver which comprisestwo radio frequency circuits tunable to the frequency of the current tobe received by said receiver, each including an inductance and acapacity, a link circuit connected across a portion of the inductance ofthe first of said tunedL circuits so selected that the terminals of saidlink circuit are at node potential relative to the current of anundesired frequency, said link circuit including an inductanceinductively related to the high potential end of the inductance ofY saidsecond tuned circuit whereby the coupling between said tuned circuitsincludes the mutual inductance between the inductance of said linkcircuit and the inductance of said second mentioned circuit and theinherent capacity between said inductances.

13. A radioreceiving apparatus of the superheterodyne type includingsignal selecting s circuits'comprising two circuits tunable to theYfrequency of the current it is desired to re- `ceive, each -includinginductances and capacities yand a link circuit effec-tively connectedacross at least a portion of the inductance of each of said circuits andincluding a capacity, the portion of the inductance' of the rst of saidtuned circuits being so chosen that the terminals of said link circuitare at node potential relative to a current of the image frequency ofsaid receiver and the portion of the inductance of the second tunedcircuit being so chosen that the voltage gain insaid circuit willcompensate for the loss due t0 the rejector action of the first circuit,and the capacity of said link circuit being so selected that saidcircuit will be resonant to a frequency outside the-band of frequencieswhich it is desired to receive.

14. A radio signal selective device fr radio receivers which comprisestwo radio frequency circuits tunable to the frequency of the current itis desired tok receive, each including an inductance and a capacity, alink circuit connected across a portion of the inductance of one of saidtuned circuits so se.

lected that the terminals of said link circuit are at node potentialrelativeto current of an Vundesired frequency and including aninductance inductively related to the high potential end of theinductance of the second of said tuned circuits whereby the'couplingbetween said tuned circuits includes the mutual inductance between theinductance of said link v,circuit and the 'inductance of said secondmentioned circuit and the inherent capacity between said inductances,the winding ofsaid inductances being so arranged that the mutualinductance coupling will oppose the capacitive couplingbetween the linkcircuit and the second of said circuits, thus providing a lesserresponse at the high frequency portion of the tuning range of saidcircuits.

15.- A radio frequency tuning system for a radio receiver of thesuperheterodyne type which comprises two circuits tunable to thefrequency of the current itis desired to receive, each circuitcomprising inductive and capacitive'reactances, and a link circuitconnected' to lsaid tuned circuits, including a portion of theinductance of each of said circuits, that portion of the inductanceofthe first of said circuits beingpso selected that the terminals ofsaid link circuit will remain at substantally node potential relative toa current of the image frequency of said receiver as the circuits aretuned, and said link circuit including an open end winding inductivelyand capacitively related to the inductance of the second of saidcircuits whereby said link circuit comprises the capacitive couplingbetween said open end winding a-nd the inductance of said second circuitand a portion of the inductance's of each ofsaid tuned circuits.

16. A radio frequency tuning system comprising two circuits tunabletothe frequency of the current it is desired to receive, each comprisinginductive and capacitive reactances, and a link circuitconnecting saidtuned circuits including a' portion of the inductance of the firstofsaid circuits so selected that the terminals of said link circuit are atnode po- -tential relative to a current of an undesired ductively andcapacitively related to the inductance of the second of said circuits,Whereby said link circuit comprises the capacitive coupling between saidopen end winding and the inductance o said second circuit and a portionof the inductance of the first circuit.

In testimony whereof I aiix my signature.

A J. KELLY JOHNSON.

